1. Field of the Invention
The present invention relates to image pickup apparatus and, more particularly, to color image pickup apparatus having white balance control means and gain or sensitivity control means.
2. Description of the Related Art
It has been proposed in color image pickup to form a luminance signal Y by sequentially switching and combining color signals, for example, R(Red), G(Green) and B(Blue) signals obtained by means of a color filter arrangement provided in front of an image pickup means, and to form color difference signals R-Y and B-Y through a matrix circuit after the color signals R, G and B have been processed by respective processing circuits. Thereafter, a standard television signal is composed from the luminance signal Y and the color difference signals R-Y and B-Y at an encoder circuit. This technique is disclosed in, for example, U.S. application Ser. No. 925,459, filed Oct. 30, 1986, now U.S. Pat. No. 4,751,567, which is a continuation of U.S. application Ser. No. 554,369, filed Nov. 22, 1983, now abandoned, assigned to the same assignee as the present application and a Japanese Laid Open (Kokai) No. Sho 53-139428 (Laid Open on Dec. 5, 1978, in Japan).
It is also known in the art to control the levels of the color signals based on the color temperature of the illumination. For this, so called "White Balancing" control, variable gain amplifiers are provided in color signal lines and the gain of each amplifier is controlled so that the ratio of the color signals becomes R:G:B=1:1:1 with respect to a white color object. This minimizes the generation of a Nyquist folding distortion and resulting false signal. This technique is disclosed in, for example, U.S. Pat. No. Re 28,774 (reissued on Apr. 13, 1976, entitled "White Balance Control System", granted to Kubota et al.).
Furthermore, it is known to increase circuit gain in order to increase sensitivity of the pickup apparatus, with some accompanying deterioration in picture quality, when the pickup apparatus is used without sufficient illumination for image pickup. Particularly, many image pickup apparatus for home use (that is, so called "Home Video" Cameras) are equipped with an automatic gain control (AGC) circuit which automatically increases the circuit gain to increase sensitivity when the object scene becomes darkened.
Here, as is commonly known, a solid state image pickup element, such as a CCD, tends to lose sensitivity to shorter wave length illumination. For example, the spectral sensitivity characteristic of a CCD image pickup element is lower for shorter wave length illumination and higher for longer wave length illumination, as shown in FIG. 5. Accordingly, the color output produced by the CCD image pickup element, though depending on the color temperature of the object, generally tends to become large for longer wave lengths (i.e., Red) and small for shorter wave lengths (i.e., Blue). Particularly, in case of an object having a low color temperature, the output for the blue color, that is, the color signal B becomes very small in comparison with the output for the red color, that is, the color signal R.
Therefore, in the case of an image pickup apparatus using a solid state image pickup element, it is necessary, at the time of forming the luminance signal (Y) by switching and combining the color signals (R, G and B), to increase the gain of the output signal of the pickup element corresponding to the shorter wave length color filter (B) in order to compensate for the low sensitivity of the solid state image pickup element in that wave length range. Thus, the gain of the color signal for the shorter wave length range (B) is further increased, for example, in order to increase sensitivity in the case of a poorly illuminated or low color temperature object. This increase in signal gain necessarily causes a lowering of the signal to noise ratio (S/N). Thus, when the gain of the color signal (B) become large, the S/N of the luminous signal (Y) lowers greatly and the resulting picture quality deteriorates.